The present invention relates to the polymerization of olefins in the presence of a bimetallic complex of nickel formed in situ.
The polymers of ethylene and other olefins are of considerable commercial attraction. The applications of these polymers are extremely numerous, from low molecular weight products for lubricants and greases, to higher molecular weight products for manufacturing fibers, films, molded articles, elastomers, etc. In the majority of cases, the polymers are obtained by catalytic polymerization of olefins by means of a transition-metal-based compound. The nature of this compound has a very strong effect on the properties of the polymer, its cost and its purity. Given the importance of polyolefins, there is a continuous need to improve the catalytic systems and to provide new ones thereof.
There is a variety of homogeneous or heterogeneous catalysts for polymerizing or copolymerizing ethylene. Among the best-known families, mention may be made, for example, of the catalysts of the xe2x80x9cZieglerxe2x80x9d type involving organometallic complexes of group III and IV metals or catalysts of the xe2x80x9cPhillipsxe2x80x9d type involving chromium complexes. However, there are also nickel-based catalysts used in particular for a number of years for producing xcex1-olefins. In addition, some systems have a degree of tolerance toward polar media.
Among the numerous catalytic systems mentioned in the literature, the association between a nickel complex, such as bis(1,5-cyclooctadiene), with benzoic acid derivatives such as 2-mercaptobenzoic acid or 3,5-diaminobenzoic acid (U.S. Pat. No. 3,637,636) or with chelating tertiary organophosphorus ligands (U.S. Pat. No. 3,635,937, U.S. Pat. No. 3,647,914) or even with glycolic, thioglycolic or thiolactic acids (U.S. Pat. No. 3,661,803) has, for example, been described. The use of a nickel complex in its zero oxidation state, such as again bis(1,5-cyclooctadiene), with a phosphorus ylid ligand, is described in U.S. Pat. No. 3,686,159. The inventions above have in common the in situ formation of the active species in the polymerization medium.
Other methods, such as in U.S. Pat. No. 4,716,205 or Bulgarian patent BG 60319 or in J. Mol. Catal. (1994), 88(2), 141-50 or in J. Mol. Catal. (1995), 103, 95-103, claim catalytic nickel systems which can be isolated, but it is necessary to introduce an accepter compound capable of extracting one of the ligands from the nickel complex in order to make it active, into the polymerization medium. The in situ technique does not allow isolation of the catalytic system so as to identify its structure accurately, but the approach has the merit of being simple and it limits the handling of catalysts, which is a so of pollution.